Control method for controlling a fuel injection system, and fuel injection system

ABSTRACT

The invention relates to a control method for controlling a fuel injection system ( 10 ) of an internal combustion engine, wherein, wherein, in a fault situation of the fuel injection system ( 10 ), a camshaft angle of a camshaft ( 34 ) which drives a pump piston ( 32 ) of a high-pressure fuel pump ( 14 ) of the fuel injection system ( 10 ) is adjusted such that an injection time (tI) of injector valve ( 42 ) which injects the fuel from the fuel injection system ( 10 ) into a combustion chamber of the internal combustion engine lies in a pressure trough ( 50 ) of a pressure oscillation in a high-pressure region ( 16 ).

FIELD OF INVENTION

The invention relates to a control method with which a fuel injectionsystem of an internal combustion engine can be operated, and to a fuelinjection system which is suitable in particular for carrying out thecontrol method.

BACKGROUND

Fuel injection systems, for example gasoline direct injection systems,have, in simplified terms, a high-pressure fuel pump, by means of whicha fuel is highly pressurized, and a high-pressure region with ahigh-pressure accumulator, the so-called rail, and with at least oneinjector valve for injecting the highly pressurized fuel into anassociated combustion chamber of an internal combustion engine. Thestated components are connected to one another by means of high-pressurelines.

For the operation of the fuel injection system, a control device, theso-called ECU, with corresponding software is normally provided. Bymeans of the control device, it is for example possible for the deliverypower of the high-pressure fuel pump to be adapted. For this purpose, onthe high-pressure fuel pump, for example, there is situated a valve,which may be formed for example as a so-called digital inlet valve. Saiddigital inlet valve may for example be provided in a “currentless open”embodiment, that is to say open when electrically deenergized, thoughother embodiments are also possible and known. Furthermore, for theregulation of the injection pressure required at the injector valves, ahigh-pressure sensor is situated in the fuel injection system, whichhigh-pressure sensor is normally attached to the high-pressureaccumulator and serves for acquiring the so-called system pressure. Inthe case of gasoline as fuel, said system pressure typically lies in arange between 150 bar and 500 bar, and in the case of diesel as fuel,said system pressure typically lies in a range between 1500 bar and 3000bar. Pressure regulation by acquisition of a signal of the high-pressuresensor, processing of the signal by means of the control device andalteration of the delivery power of the high-pressure fuel pump by meansof the digital inlet valve is normally performed. The high-pressure fuelpump is normally mechanically driven by the internal combustion engineitself, for example by means of a camshaft.

In the described high-pressure fuel pumps with a digital inlet valve,faults may arise which lead to an undesirably increased delivery powerof the high-pressure fuel pump. This may for example be caused by theinlet valve on the high-pressure fuel pump no longer being able to befully opened or closed. It is for example also conceivable that, forexample as a result of a spring breakage at a spring in the inlet valve,or further possible faults, the delivery power can no longer becontrolled.

In such a fault situation, a volume flow for the high-pressure fuel pumpis set in a manner dependent on the rotational speed of the internalcombustion engine and the temperature prevailing in the fuel injectionsystem. Here, said volume flow may be greater than the injectionquantity of the at least one injector valve. For example, in a typicaloperating state, the so-called overrun mode of the internal combustionengine, no or only little injection is performed through the injectorvalve. Therefore, if the high-pressure fuel pump delivers an excessivelylarge volume flow, an undesired pressure increase occurs in the fuelinjection system.

To be able to deplete undesirably high pressures in the high-pressureregion of the fuel injection system, it is common for a mechanicalsafety valve, a so-called pressure-limiting valve, to be provided on thehigh-pressure fuel pump, which valve can limit or restrict the pressure.

Typical p-Q characteristics of the pressure-limiting valve areconfigured such that a maximum pressure takes effect in thehigh-pressure accumulator, which maximum pressure exceeds the nominalpressures of the injector valve during normal operation.

After the fault situation, the pressure increases within a few pumpstrokes of the high-pressure fuel pump up to a maximum pressure, whichtakes effect in the high-pressure region.

The pressure-limiting valve is commonly designed so as to discharge intoa pressure chamber of the high-pressure fuel pump, such that saidpressure-limiting valve is hydraulically blocked during a delivery phaseof the high-pressure fuel pump. This means that the pressure-limitingvalve can open, and discharge fuel out of the high-pressure region,exclusively in the suction phase of the high-pressure fuel pump. Suchpressure-limiting valves are referred to as hydraulically blockedpressure-limiting valves.

Owing to the structural nature of the injector valve, the injector valvecommonly opens counter to the pressure prevailing in the high-pressureaccumulator. Here, in a manner dependent on the operating state of theinternal combustion engine, an actuation profile is used for theactuation of the injector valve in order to open the injector valve suchthat an injection can begin.

Many injector valves are designed not for the maximum pressure in thefault situation but, in a cost-optimized manner, for normal operation.In this way, in fault situations with excessively high pressures in thehigh-pressure region, the injector valve can no longer open, and theinternal combustion engine can thus no longer operate. This can resultin a breakdown of a vehicle operated with the internal combustionengine.

SUMMARY

It is therefore an object of the invention to propose a control methodfor operating a fuel injection system, and a corresponding fuelinjection system, by means of which a failure of the internal combustionengine can be prevented even in a fault situation.

This object is achieved by a control method having the features ofindependent claim 1.

The coordinate claim relates to a fuel injection system which isdesigned in particular for carrying out the control method.

Advantageous configurations of the invention are the subject of thedependent claims.

In a control method for controlling a fuel injection system of aninternal combustion engine, it is firstly the case that a fuel injectionsystem is provided, which has a high-pressure fuel pump with a pumppiston which moves between a bottom dead center and a top dead center ina pressure chamber during operation and which serves for highlypressurizing a fuel, a camshaft for driving the pump piston, and ahigh-pressure region with at least one injector valve which serves forinjecting highly pressurized fuel into a combustion chamber of aninternal combustion engine.

Furthermore, a pressure-limiting valve is provided which, when apredefined opening pressure is reached in the high-pressure region,discharges fuel from the high-pressure region into the pressure chamberof the high-pressure fuel pump. A fault situation in the fuel injectionsystem is detected, wherein the fault situation lies in the fact thatthe predefined opening pressure is overshot in the high-pressure region.Furthermore, a period duration with four evenly distributed quadrants isdetermined between a first TDC time, at which the pump piston is at thetop dead center, and a second TDC time, at which the pump piston is atthe top dead center. An injection time at which the injector valvebegins to inject fuel is set. A camshaft adjuster is provided in thefuel injection system for the purposes of enabling a camshaft angle ofthe camshaft relative to the pump piston to be adjusted. After theinjection time is set, the camshaft angle of the camshaft is adjustedsuch that the injection time lies in a duration which extends in asecond quadrant of the period duration and/or in a third quadrant of theperiod duration.

In the fault situation, the pressure in the high-pressure regionincreases continuously within a few strokes of the pump piston. In thiscase, the pressure-limiting valve discharges fuel from the high-pressureregion into the pressure chamber of the high-pressure fuel pump, but ishydraulically locked in the delivery phase of the high-pressure fuelpump, such that the pressure in the high-pressure region increases atthis time, and falls again when the pressure-limiting valve is opened.In the fault situation, therefore, a pressure oscillation forms in thehigh-pressure region, with pressure troughs, specifically when thepressure-limiting valve can discharge fuel into the pressure chamber,and pressure peaks, when the pressure-limiting valve is hydraulicallyblocked. Here, the pressure troughs correspond to bottom dead centers ofthe pump piston, whereas the pressure peaks correspond to top deadcenters of the pump piston. The time at which the pump piston issituated at the top dead center or at the bottom dead centerrespectively is dependent on a camshaft angle of the camshaft relativeto the pump piston. If this camshaft angle is adjusted, the time atwhich the pump piston is situated at the top dead center or at thebottom dead center respectively changes. If it is now set in apredefined manner when an injector valve is to inject fuel into acombustion chamber, it is possible, through adjustment of the camshaftangle of the camshaft relative to the pump piston, for a pressure troughto be set at a time such that the injection time of the injector valvefalls exactly into said pressure trough. For this purpose, it mustinitially be determined beforehand how the period duration, that is tosay a duration between two adjacent top dead centers of the pump piston,is formed. Said period duration is then divided into four equally sizedquadrants. Here, a pressure trough is situated exactly between thesecond quadrant and the third quadrant. The camshaft angle is adjustedsuch that the pressure trough and thus the second and third quadrantcome to lie, in terms of time, such that the injection time is situatedtherein.

Therefore, if the high-pressure fuel pump is driven mechanically bymeans of a camshaft, wherein the camshaft exhibits an adjustment of thecamshaft angle, for example by means of a camshaft adjuster, which maybe hydraulically or electrically driven, then in the event of a faultsituation being detected, the camshaft is adjusted such that theinjection time falls into the negative amplitude, that is to say intothe pressure trough of the pressure oscillation in the high-pressureregion. In this way, the injector valve can still open even if theaveraged pressure in the high-pressure region is above a pressurecritical for the injector opening.

In an advantageous embodiment, the fault situation is detected by meansof a high-pressure sensor arranged in the high-pressure region. Suchhigh-pressure sensors are provided in the high-pressure region of thefuel injection system in any case and can therefore be used as signaltransmitters for the control of the fuel injection system in the faultsituation.

The opening pressure of the pressure-limiting valve is advantageouslyset to be lower than a maximum admissible maximum pressure in thehigh-pressure region. Here, the maximum pressure is to be understood tomean a pressure in the high-pressure region counter to which theinjector valves can still just open. For example, the maximum pressuremay be defined in a range above 500 bar. It is advantageous for theopening pressure of the pressure-limiting valve to be considerably lowerthan this, in order to thereby prevent a maximum pressure from formingin the high-pressure region in the first place. Advantageous pressureranges of the opening pressure of the pressure-limiting valve lie inthis case in a range between 300 bar and 400 bar, wherein the openingpressure already exceeds the nominal pressures in the normal mode of theinjector valves. Said nominal pressures normally lie in a range between200 bar and 280 bar.

Preferably, the injection time is set in a manner dependent on a fueldemand from the internal combustion engine. That is to say that fuel isinjected only when the internal combustion engine actually requires fuelfor the operation thereof.

Advantageously, upon a detection of re-entry into a normal mode of thefuel injection system in which the predefined opening pressure isundershot again in the high pressure region, the adjustment of thecamshaft is ended in a manner dependent on the set injection time.

In an advantageous refinement, a characteristic map is stored, whichassigns every camshaft angle of the camshaft relative to the pump pistona predetermined TDC time. It is therefore possible to adjust thecamshaft angle in targeted fashion if the four quadrants of the periodduration are known in terms of their temporal position.

In an advantageous configuration, at least two operating states of theinternal combustion engine are provided, wherein, in an overrun mode, noinjection of fuel through the injector valve into the combustion chambertakes place, wherein, in an injection mode, at least one injection offuel through the injector valve into the combustion chamber takes place.In the fault situation, the overrun mode of the internal combustionengine is deactivated, such that the internal combustion engine isoperated exclusively in the injection mode. In this way, the openingcapability of the injector valve can be additionally assisted, becausephases are avoided in which no fuel whatsoever is extracted from thehigh-pressure region and thus the pressure in the high-pressure regioncould build up further. In general, the fault situation is not relevantto the exhaust gas, and a possible power loss is acceptable in the faultsituation, because in this way, it is basically possible to preventfailure of the internal combustion engine and a breakdown of a vehicleoperated with the internal combustion engine.

When the overrun mode is deactivated, it is advantageous if such aquantity of fuel is injected through the injector valve in the injectionmode that a high pressure which is lower than the maximum pressure andwhich altogether advantageously corresponds to an opening pressure ofthe pressure-limiting valve is set in the high-pressure region. As aresult, the injector valves can continue to be reliably opened.

A fuel injection system for injecting fuel into combustion chambers ofan internal combustion engine is designed in particular for carrying outthe control method described above. Here, the fuel injection system hasa high-pressure fuel pump with a pump piston which moves between abottom dead center and a top dead center in a pressure chamber duringoperation and which serves for highly pressurizing a fuel. Furthermore,a camshaft is provided which serves for driving the pump piston andwhich has a camshaft adjuster for the adjustment of a camshaft angle ofthe camshaft relative to the pump piston. Furthermore, the fuelinjection system comprises a high-pressure region with at least oneinjector valve for injecting highly pressurized fuel into a combustionchamber of the internal combustion engine. Furthermore, apressure-limiting valve is provided which is arranged in thehigh-pressure region and which is designed to, when a predefined openingpressure is reached in the high-pressure region, discharge a fuel fromthe high-pressure region into the pressure chamber of the high-pressurefuel pump. Furthermore, the fuel injection system comprises a controldevice which is designed to detect a fault situation in the fuelinjection system, wherein the fault lies in the fact that the predefinedopening pressure is overshot in the high-pressure region. Furthermore,the control device is designed to determine a period duration with fourevenly distributed quadrants between a first TDC time, at which the pumppiston is at the top dead center, and a second TDC time, at which thepump piston is at the top dead center. The control device is furthermoredesigned to set an injection time at which the injector valve begins toinject fuel, and to adjust the camshaft angle of the camshaft such thatthe injection time lies in a duration which extends in a second quadrantof the period duration and/or in a third quadrant of the periodduration.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantageous configurations of the invention will be discussed in moredetail below on the basis of the appended drawings, in which:

FIG. 1 is a schematic illustration of a fuel injection system forinjecting fuel into combustion chambers of an internal combustionengine;

FIG. 2 shows a pressure-time diagram which illustrates a pressureoscillation in a high-pressure region of the fuel injection system fromFIG. 1 in a fault situation;

FIG. 3 shows a flow diagram which schematically illustrates an operatingmethod for operating the fuel injection system from FIG. 1 in the faultsituation, in a first embodiment;

FIG. 4 is a schematic illustration of a control device which is designedfor carrying out the operating method as per FIG. 3;

FIG. 5 shows a flow diagram which schematically illustrates an actuationmethod for the actuation of the fuel injection system from FIG. 1 in afault situation, in a second embodiment;

FIG. 6 is a schematic illustration of a control device which is designedfor carrying out the actuation method as per FIG. 5;

FIG. 7 shows a flow diagram which schematically illustrates an actuationmethod for the actuation of an injector valve of the fuel injectionsystem from FIG. 1 in a fault situation of the fuel injection system;and

FIG. 8 shows a control device which is designed for carrying out theactuation method as per FIG. 7.

DETAILED DESCRIPTION

FIG. 1 shows a fuel injection system 10 by means of which fuel can beinjected into combustion chambers of an internal combustion engine. Forthis purpose, the fuel injection system 10 has a fuel accumulator 12such as for example a tank, a high-pressure fuel pump 14, and ahigh-pressure region 16 situated downstream of the high-pressure fuelpump 14. From the fuel accumulator 12, fuel is pumped for example bymeans of a tank pump 18 into a low-pressure line 20 and is thusdelivered to a pressure chamber 22 of the high-pressure fuel pump 14. Tobe able to regulate a delivery power of the high-pressure fuel pump 14,a digital inlet valve 24 is connected upstream of the pressure chamber22 in the low-pressure line 20. Said digital inlet valve 24 may beactuated by a control device 26 in order to regulate the fuel quantitythat is highly pressurized by the high-pressure fuel pump 14 in thepressure chamber 22. Additional elements such as filters 28 and anevaporator 30 are arranged in the low-pressure line 20 in order topurify the fuel from the fuel regulator 12 and also dampen pulsationdamping actions in the low-pressure line 20.

A pump piston 32 moves in translational fashion back and forth in thepressure chamber 22, and in so doing increases and decreases the volumeof the pressure chamber 22. The pump piston 32 is driven in itstranslational movement by a camshaft 34. Here, the camshaft 34 iscoupled for example to a crankshaft of the internal combustion engineand is thus driven by the internal combustion engine itself. During themovement of the pump piston 32 in the pressure chamber 22, the pumppiston 32 reaches a top dead center TDC at the moment at which thepressure chamber 22 has its smallest volume, and reaches a bottom deadcenter BDC at the moment at which the pressure chamber 22 reaches itslargest volume. The corresponding times are thus the TDC time and theBDC time.

Highly pressurized fuel is then released via an outlet valve 36 from thehigh-pressure fuel pump 14 into the high-pressure region 16 and isconducted via a high-pressure line 38 to a pressure accumulator 40, inwhich the highly pressurized fuel is stored until it is injected viainjector valves 42, which are arranged on the pressure accumulator 40,into combustion chambers of an internal combustion engine.

To regulate the delivery power of the high-pressure fuel pump 14, thereis arranged on the pressure accumulator 40 a high-pressure sensor 44which monitors the pressure prevailing in the pressure accumulator 40.The high-pressure sensor 40 transmits a signal to the control device 26,which then actuates the inlet valve 24 in a manner dependent on thissignal, such that the high pressure in the pressure accumulator 40 canbe regulated.

In a fault situation, it may be the case that the high-pressure fuelpump 14 has an increased delivery power, and thus a pressure isgenerated in the pressure accumulator 40 which is much higher than anormal pressure during normal operation. For this situation, apressure-limiting valve 46 is provided on the high-pressure line 38,which pressure-limiting valve discharges fuel from the high-pressureregion 16 in order to thereby lower the pressure in the high-pressureregion 16. Here, the pressure-limiting valve 46 discharges the fuel intothe pressure chamber 22 of the high-pressure fuel pump 14. Since thepressure-limiting valve 46 is normally formed as a check valve, thepressure-limiting valve 46 is hydraulically locked when thehigh-pressure fuel pump 14 is in the delivery phase, that is to say whenfuel in the pressure chamber 22 is highly pressurized and is thendischarged via the outlet valve 36 into the high-pressure region 16.However, if the high-pressure fuel pump 14 is situated in a suctionphase, the pump piston 32 moves towards its bottom dead center BDC, thevolume in the pressure chamber 22 is expanded, and the pressure-limitingvalve 46 can open and discharge fuel into the pressure chamber 22.

Here, an opening pressure P_(open) is set so as to be lower than amaximum admissible maximum pressure P_(max) in the high pressure region16 at which it is still just possible for the injector valves 42 to opencounter to said high pressure and inject fuel into the combustionchambers. For example, such a maximum pressure P_(max) lies above 500bar. The opening pressure P_(open) of the pressure-limiting valve 46 isthus advantageously set in a range between 300 bar and 500 bar. Thisexceeds the nominal pressures of approximately 250 bar during normaloperation, in the case of which the injector valves 42 can be operatedwithout problems.

In a fault situation as described above, for example as a result of aspring breakage at the inlet valve 24 or other fault situations thatprevent regulation of the pump delivery power, the high-pressure fuelpump 14 passes into the state of so-called full delivery, and deliversfuel unhindered into the high-pressure region 16. Since thepressure-limiting valve 46 can discharge the fuel into the pressurechamber 22 only during the suction phase of the high-pressure fuel pump14, the high pressure in the high-pressure region 16 increases within afew pump strokes to a maximum which takes effect.

This will be briefly discussed with reference to the diagram in FIG. 2.Here, the diagram illustrates a pressure-time diagram, wherein apressure p in the high-pressure region 16 is plotted versus a time t inwhich the high-pressure fuel pump 14 performs pump strokes.

Here, the fault situation occurs at a time t₁. As can be seen, thepressure p in the high-pressure region 16 increases continuously afterthis time t₁ until the opening pressure P_(open) of thepressure-limiting valve 46 is reached at a time t₂.

Here, the diagram in FIG. 2 shows the pressure build-up after a faultsituation in which the high-pressure fuel pump 14 is set into a fulldelivery position. The speed with which the opening pressure P_(open) ofthe pressure-limiting valve 46 is reached is dependent on the rotationalspeed of the high-pressure fuel pump 14, which is dependent on arotational speed of the crankshaft of the internal combustion engine.Furthermore, the pressure increase is also dependent on the temperaturein the fuel injection system 10. Here, FIG. 2 illustrates a situation inwhich the internal combustion engine is in the overrun mode, that is tosay in an operating state in which no injection of fuel through theinjector valve 42 into the combustion chamber occurs.

Since the pressure-limiting valve 46 can discharge into the pressurechamber 22 only when the pressure in the pressure chamber 22 is lowerthan in the high-pressure region 16, a pressure oscillation occurs inthe high-pressure region 16, which is distinguished by the fact that,during the discharging of the pressure-limiting valve 46, the highpressure in the high-pressure region 16 falls and then increases againif the pressure-limiting valve 46 is hydraulically blocked. Owing to theembodiment of the pressure-limiting valve 46 as a hydraulically blockedpressure-limiting valve, the characteristic shown in FIG. 2 is thusrealized, with pressure peaks 48 when the high-pressure fuel pump 14 isin the delivery phase and with pressure troughs 50 when thehigh-pressure fuel pump 14 is in the suction phase.

If a fault situation arises which leads to overdelivery or full deliveryof the high-pressure fuel pump 14, the maximum pressure in the pressureaccumulator 40 therefore increases, in particular in the overrun mode orin operating states with a low injection quantity, in a manner dependenton the present rotational speed of the internal combustion engine andthe temperature in the fuel injection system 10. In the case ofpressures higher than the maximum admissible injector opening pressureP_(max), misfiring of the internal combustion engine or even a breakdownof a vehicle operated with the internal combustion engine can occur.

To prevent the pressure that prevails at the injector valves 42 fromincreasing beyond the maximum pressure P_(max) at which the injectorvalves 42 still open, the methods described below can be carried out.Below, three different methods will be described, which can beimplemented as countermeasures; the methods may be implemented in eachcase individually or in combination. The control device 26 is in eachcase designed to carry out each of said methods. If the methods arecarried out simultaneously, the control device 26 is configuredcorrespondingly.

Below, however, for the sake of clarity, the methods will be describedonly as methods to be carried out individually.

A first countermeasure with which a shutdown of the internal combustionengine can be prevented is in this case a so-called overrundeactivation, which will be described below with reference to FIG. 3 andFIG. 4.

Here, FIG. 3 schematically shows, on the basis of a flow diagram, thesteps of an operating method with which such overrun deactivation can beimplemented, whereas FIG. 4 schematically shows the control device 26that is configured for carrying out the operating method as per FIG. 3.

The internal combustion engine is operated by the control device 26 inat least two operating states, specifically in an overrun mode and in aninjection mode. Here, in the overrun mode, no fuel is injected via theinjector valves 42 into the combustion chambers of the internalcombustion engine, whereas, in the injection mode, at least oneinjection of fuel through the injector valves 42 into the combustionchambers occurs.

In the operating method, in a first step, a pressure p in thehigh-pressure region 16 is firstly acquired by means of thehigh-pressure sensor 44. For this purpose, the control device 26 has apressure acquisition device 52, which communicates with thehigh-pressure sensor 44. The opening pressure P_(open) of thepressure-limiting valve 46 is also stored in the control device 26.

In a subsequent step of the operating method, it is thereforedetermined, by means of a fault detection device 54 of the controldevice 26, whether the pressure p is higher than or equal to the openingpressure P_(open) of the pressure-limiting valve 46. If this is thecase, the fault detection device 54 detects that a fault situation ispresent. In this case, the overrun mode of the internal combustionengine is deactivated by an overrun deactivation device 56 in thecontrol device 26. This means that an overrun deactivation of theinjector valves 42, such that they inject no further fuel into theinternal combustion engine, is prohibited, and only fired overrun, thatis to say the injection mode of the internal combustion engine, ispermitted by the control device 26. It is thereby ensured that always acertain fuel quantity is discharged via the injector valves 42 and thusextracted from the high-pressure region 16. The pressure level in thehigh-pressure region 16 is in this case kept below the critical pressureP_(max) for the injector opening, and is preferably even lowered to suchan extent as to lie in the range of the opening pressure P_(open) of thepressure-limiting valve 46.

After detection of the fault situation that leads to the uncontrolleddelivery by the high-pressure fuel pump 14, therefore, the overrun mode,in which no fuel is injected, is prohibited, and instead, only anoperating state with an at least small injection quantity is permittedand also implemented. The corresponding function is in this case storedin the control device 26.

If it is however identified in the operating method that the pressure pin the high-pressure region 16 is not higher than or equal to theopening pressure P_(open) of the pressure-limiting valve 46, the faultdetection device 54 identifies that no fault situation is present, andthe overrun mode of the internal combustion engine remains permitted.Both after permission of the overrun mode and after deactivation of theoverrun mode, it is always the case that the pressure p in thehigh-pressure region 16 is acquired again and it is checked whether saidpressure is higher than or equal to the opening pressure P_(open) of thepressure-limiting valve 46.

If the situation arises in which, after deactivation of the overrunmode, the pressure p in the high-pressure region 16 has fallen below theopening pressure P_(open), the fault detection device 54 detects thatthe fuel injection system 10 has entered a normal mode again. In thiscase, the overrun mode can then be reactivated. This means that thefunctionality can be optionally withdrawn again in a manner dependent onthe pressure conditions in the fuel injection system 10.

Altogether, by means of the operating method, the risk of a breakdown ofa vehicle operated with the internal combustion engine is reduced. Here,the fault situation is not relevant to the exhaust gas. A possible powerloss is acceptable in the fault situation.

An actuation method for actuating the fuel injection system 10, whichmay be carried out alternatively or in addition to the overrundeactivation described above, will be described below with reference toFIG. 5 and FIG. 6. Here, a camshaft angle of the camshaft 34 relative tothe pump piston 32 is adjusted in targeted fashion by means of acamshaft adjuster 58 provided in the fuel injection system 10.

The camshaft 34 rotates about a camshaft axis 60, wherein, at regularintervals, a cam 52 comes into contact with the pump piston 32 such thatthe pump piston 32 is moved toward the top dead center TDC. As thecamshaft 34 rotates onward, the cam 62 moves away from the pump piston32 again, and the pump piston 32 moves in the direction of the bottomdead center BDC. Therefore, in periodic intervals, the pump piston 32,moved by the cam 62, is situated alternately at the top dead center TDCand at the bottom dead center BDC. However, if an angle between pumppiston 32 and the camshaft 34 is adjusted during the operation of thecamshaft 34, the spacing between two successive top dead centers TDC isno longer uniform, as illustrated for example in the diagram shown inFIG. 2, it rather being the case that the TDC time of the top deadcenter TDC changes.

The adjustment of the angle of the camshaft 34 may likewise be inducedby means of the control device 26, by means of a cam angle adjustmentdevice 64 arranged in the control device 26.

If an injection time t_(I) at which the injector valves 42 begin theinjection of fuel into the combustion chambers is known, for example byvirtue of an opening time t_(open) for the injector valves 42 being setby means of an opening time setting device 66 in the control device 26,the camshaft 34 can be adjusted by means of the camshaft angleadjustment device 64 such that the injection time t_(I) is situated inthe pressure trough shown in FIG. 2.

For this purpose, as per the flow diagram in FIG. 5, it is firstly thecase that a period duration t_(p) of the pressure oscillation in thehigh-pressure region 16 is determined. Here, the period duration t_(p)corresponds to a duration between the time at which the pump piston 32reaches a first top dead center TDC and a time at which the pump piston32 next reaches a top dead center. Owing to the mechanical connection ofthe high-pressure fuel pump 14 to the internal combustion engine, theposition of the camshaft 34 and thus of the top dead center TDC of thepump piston 32 are known and are stored in a first characteristic map K1in the control device 26, wherein the characteristic map K1 assignsevery crankshaft angle a position of the pump piston 32. Also arrangedin the control device 26 is a crank angle acquisition device 68 by meansof which the control device 26 can acquire the present crankshaft angle.A TDC detection device 70 can therefore, from the data of the firstcharacteristic map K1 and the data of the crankshaft acquisition device68, detect when the pump piston 32 is situated at a top dead center TDC.This information is fed to an evaluation device 72 which is arranged inthe control device 26 and which, from said information, determines theperiod duration t_(p). Furthermore, the evaluation device 72 divides theperiod duration TP into four evenly distributed quadrants Q1, Q2, Q3 andQ4.

In the actuation method, it is subsequently, analogously to the overrundeactivation, identified whether a fault situation is present in thefuel injection system 10. If a fault situation is present, there isfirstly a waiting period until a fuel demand detection device 74 detectswhether a fuel demand from the internal combustion engine is present,that is to say whether an injection via the injector valves 42 isrequired. If this is the case, the injection time t_(I) is firstly setto an arbitrary time. Then, by means of the camshaft adjuster 58, whichis driven by the camshaft angle adjustment device 64, an angle of thecamshaft relative to the pump piston 32 is adjusted such that thepreviously set injection time t_(I) falls into the pressure trough ofthe pressure oscillation from FIG. 2, that is to say into the durationof the second quadrant Q2 or of the third quadrant Q3.

However, if no fuel demand is present, no injection via the injectorvalves 42 is performed.

To be able to adjust the camshaft angle in targeted fashion, a secondcharacteristic map K2 is stored in the control device 26, which secondcharacteristic map assigns every camshaft angle of the camshaft 34relative to the pump piston 32 a predetermined time at which the pumppiston 32 is situated at the top dead center TDC. Also arranged in thecontrol device 26 is a memory device 76 which stores the presentcamshaft angle. The data of the characteristic map K2 and of the memorydevice 76 are fed to the camshaft angle adjustment device 64, in orderthat the camshaft angle can be adjusted in targeted fashion.Furthermore, the camshaft angle adjustment device 64 outputs a signal tothe camshaft adjuster 58 only if the information regarding when theinjection through the injector valves 42 is supposed to start ispresent, that is to say when the injection time t_(I) has been set. Thecamshaft adjuster 58 adjusts the angle of the camshaft 34 only when afault situation is actually present, wherein the camshaft angleadjustment device 64 is additionally fed with the information from theevaluation device 72 as regards where the pressure trough 50 ispresently situated.

If the fault detection device 54 identifies that no fault situation ispresent, and if the fuel demand detection device 74 detects that fuel isdemanded by the internal combustion engine, fuel is injected entirelynormally via the injector valves 42 into the respective combustionchambers. In the absence of a fuel demand, however, the injector valves42 do not open.

The method in which the camshaft angle is adjusted in order to therebyshift the injection time t_(I) into a pressure trough 50 is also carriedout continuously in order to thereby detect whether the fuel injectionsystem 10 has entered a normal mode and the pressure p in thehigh-pressure region 16 lies below the opening pressure P_(open) again.In this case, the adjustment of the camshaft 34 is ended in a mannerdependent on the set injection time t_(I).

Therefore, if the high-pressure fuel pump 14 is mechanically driven bymeans of a camshaft 34 which exhibits a means for adjusting the angle,that is to say a so-called camshaft adjuster 58, which may behydraulically or electrically operated, then in the event of a faultsituation being detected, the camshaft 34 is adjusted by means of thecamshaft adjuster 58 such that the start of injection, that is to saythe injection time t_(I), falls into the negative amplitude, that is tosay into the pressure trough 50, of the rail pressure oscillation as perFIG. 2. Therefore, the injector valves 42 can still open even if theaveraged pressure in the pressure accumulator 40 lies above the pressureP_(max) critical for the injector opening. Therefore, a functionality isproposed by means of which an adjustment of the camshaft 34 by means ofthe camshaft adjuster 58 is possible, such that the start of injectionof the injector valves 42 is relocated into regions expedient withregard to pressure, specifically the pressure troughs 50. This functionis also stored in the control device 26, and the functionality mayoptionally be withdrawn again in a manner dependent on the pressureconditions in the fuel injection system 10.

Below, with regard to FIG. 7 and FIG. 8, a third method will bedescribed with which it is sought for an opening of the injector valves42 to remain possible even in the fault situation of the fuel injectionsystem 10. This method may be carried out in addition to the overrundeactivation and as an alternative to the adjustment of the camshaft 34.Here, too, the phenomenon is utilized whereby an injector valve 42 thatseeks to open during a pressure peak 48 must open counter to a higherpressure than if it were to do so in a pressure trough 50. Thedifference between the pressure peak 48 and the pressure trough 50 issystem-dependent, and may amount to for example 50 bar.

If the respective injector valve 42 opens in a pressure trough 50, thetemperature and rotational speed range in which operation of theinternal combustion engine is possible is expanded in relation to theinjection during the pressure peak 48. Alternatively, a less expensiveor more robust design of the pressure-limiting valve 46 may also beused, with the result of higher maximum pressures P_(max), and undersome circumstances exhibit comparable exhibited operation of theinternal combustion engine.

As already described, the pressure peak 48 in the high-pressure region16 correlates with the top dead center TDC of the high-pressure fuelpump 14, wherein the propagation time of the fuel through the fuelinjection system 10 proceeding from the outlet valve 36 mustadditionally be observed. Owing to the mechanical connection of thehigh-pressure fuel pump 14 to the internal combustion engine, saidposition of the top dead center TDC is known. As is also the case in theother methods, the fault situation is detected by detection of anundesirably high pressure in the high-pressure region 16 by means of thehigh-pressure sensor 44.

The start of injection of the injector valves 42 is stored in thecontrol device 26 as a characteristic map.

As in the case of the method for the adjustment of the camshaft angle,the period duration tp between two TDC points of the pump piston 32 isdetermined, and the period duration TP is divided into four equallysized quadrants Q1 to Q4. Here, the injector valves 42 are actuated suchthat the opening time T_(open) of the injector valves 42 lies in anopening duration which extends into the second quadrant Q2 and into thethird quadrant Q3. This means that the camshaft 34 is not adjusted, butrather the opening time T_(open) of the injector valves 42 is activelyshifted. By shifting the opening time T_(open) into the pressure trough50 specifically only after detection of the fault situation, thedescribed advantages can be utilized. The shift of the opening timeT_(open) during operation of the internal combustion engine is notrelevant to emissions, because it is a fault situation.

Therefore, in the method, as in the case of the adjustment of thecamshaft 34, the period duration tp is firstly determined, and it isthen detected whether or not a fault situation is present.

In this case, too, the injector valves 42 are actuated only when a fueldemand from the internal combustion engine is actually present. If thisis the case, the opening time T_(open) is shifted into the secondquadrant Q2 or third quadrant Q3 of the period duration tp. However, ifno fuel demand is present, no injection occurs.

After the shift of the opening time T_(open), it is in turn checkedwhether the fuel injection system 10 remains in a fault situation,because it is optionally possible in this case too for the functionalityto be withdrawn again if the fuel injection system 10 enters the normalmode again. In this case, the injection in the period duration tp occursas desired in any of the four quadrants Q1 to Q4 directly in accordancewith a fuel demand from the internal combustion engine.

Therefore, in the control device 26, a functionality is stored which,after the detection of a fault situation with an associated pressureincrease in the high-pressure region 16, shifts the existing openingtime T_(open) of the injector valves 42 for normal operation into arange which is more optimum for emergency running of the internalcombustion engine. For this purpose, in the control device 26, acorresponding characteristic map may be stored, for example in the formof the opening time setting device 66, which shifts the opening timeT_(open) of the injector valves 42 such that it lies in the pressuretrough 50. The characteristic map may optionally be configured as afunction of pressure and/or temperature and/or rotational speed of theinternal combustion engine.

The shift of the opening time T_(open) may optionally be withdrawn againin a manner dependent on the pressure conditions in the system.

The invention claimed is:
 1. A control method for controlling a fuelinjection system of an internal combustion engine, the methodcomprising: providing a fuel injection system having a high-pressurefuel pump with a pump piston which moves between a bottom dead centerand a top dead center (TDC) in a pressure chamber during operation andwhich serves for highly pressurizing a fuel, having a camshaft fordriving the pump piston and having a high-pressure region with at leastone injector valve for injecting highly pressurized fuel into acombustion chamber of an internal combustion engine; providing apressure-limiting valve which, when a predefined opening pressure isreached in the high-pressure region discharges fuel from thehigh-pressure region into the pressure chamber of the high-pressure fuelpump; detecting a fault situation in the fuel injection system, whereinthe predefined opening pressure is overshot in the high-pressure region;determining a period duration with four evenly distributed quadrantsbetween a first TDC time, at which the pump piston is at the top deadcenter (TDC), and a second TDC time, at which the pump piston is at thetop dead center; setting an injection time at which the injector valvebegins to inject fuel; providing a camshaft adjuster for adjusting acamshaft angle of the camshaft relative to the pump piston; adjustingthe camshaft angle of the camshaft such that the injection time lies ina duration which extends in a second quadrant of the period durationand/or in a third quadrant of the period duration.
 2. The method ofclaim 1, wherein the fault situation is detected by a high-pressuresensor arranged in the high-pressure region.
 3. The method of claim 1,wherein the opening pressure of the pressure-limiting valve is set to belower than a maximum admissible maximum pressure in the high-pressureregion and wherein the maximum pressure is defined in particular in arange above 500 bar.
 4. The method of claim 1, wherein: the injectiontime is set dependent on a fuel demand from the internal combustionengine.
 5. The method of claim 1, wherein: upon detection of re-entryinto a normal mode of the fuel injection system in which the predefinedopening pressure is undershot again in the high-pressure regionadjustment of the camshaft is ended in a manner dependent on the setinjection time.
 6. The method of claim 1, wherein, a characteristic mapis stored, which assigns every camshaft angle of the camshaft relativeto the pump piston at a predetermined TDC time.
 7. The method of claim1, wherein at least two operating modes of the internal combustionengine are provided, wherein, in an overrun mode, no injection of fuelthrough the injector valve into the combustion chamber takes place,wherein, in an injection mode, at least one injection of fuel throughthe injector valve into the combustion chamber takes place, wherein, inthe fault situation, the overrun mode of the internal combustion engineis deactivated, such that the internal combustion engine is operatedexclusively in the injection mode.
 8. The method of claim 7, wherein, aquantity of fuel is injected through the injector valve at a highpressure which is lower than the maximum pressure and which correspondsto an opening pressure of the pressure-limiting valve in thehigh-pressure region.
 9. A fuel injection system for injecting fuel intocombustion chambers of an internal combustion engine, the systemcomprising: a high-pressure fuel pump with a pump piston which movesbetween a bottom dead center and a top dead center (TDC) in a pressurechamber during operation and which serves for highly pressurizing afuel; a camshaft which drives the pump piston and which has a camshaftadjuster configured to adjust a camshaft angle of the camshaft relativeto the pump piston; at least one injector valve-configured to injectpressurized fuel into a combustion chamber of the internal combustionengine from a high-pressure region; a pressure-limiting valve configuredto discharge fuel from the high-pressure region into the pressurechamber of the high-pressure fuel pump when a predefined openingpressure is reached in the high-pressure region; and a control deviceconfigured to: detect a fault situation in the fuel injection systemwhen the predefined opening pressure is overshot in the high-pressureregion and, determine a period duration with four evenly distributedquadrants between a first TDC time, at which the pump piston is at thetop dead center, and a second TDC time, at which the pump piston is atthe top dead center, and set an injection time at which the injectorvalve begins to inject fuel; and adjust the camshaft angle of thecamshaft relative to the pump piston such that the injection time liesin a duration which extends in a second quadrant of the period durationand/or in a third quadrant of the period duration.
 10. The fuelinjection system of claim 9, wherein the fault situation is detected bya high-pressure sensor arranged in the high-pressure region.
 11. Thefuel injection system of claim 9, wherein the opening pressure of thepressure-limiting valve is set by the control device to be lower than amaximum admissible maximum pressure in the high-pressure region andwherein the maximum pressure is defined in particular in a range above500 bar.
 12. The fuel injection system of claim 9, wherein the injectiontime is set by the control device dependent on a fuel demand from theinternal combustion engine.
 13. The fuel injection system of claim 9,wherein: upon detection by the control device of re-entry into a normalmode of the fuel injection system in which the predefined openingpressure is undershot again in the high-pressure region adjustment ofthe camshaft is ended in a manner dependent on the set injection time.14. The fuel injection system of claim 9, wherein, a characteristic mapis stored in memory in the control device, which assigns every camshaftangle of the camshaft relative to the pump piston at a predetermined TDCtime.
 15. The fuel injection system of claim 9, wherein at least twooperating modes of the internal combustion engine are utilized, wherein,in an overrun mode, no injection of fuel through the injector into thecombustion chamber takes place, wherein, in an injection mode, at leastone injection of fuel through the injector valve into the combustionchamber takes place, and wherein, in the fault situation, the overrunmode of the internal combustion engine is deactivated, such that theinternal combustion engine is operated exclusively in the injectionmode.
 16. The fuel injection system of claim 15, wherein, a quantity offuel is injected through the injector valve at a high pressure which islower than the maximum pressure and which corresponds to an openingpressure of the pressure-limiting valve in the high-pressure region.